The flu has killed some 6,750 people around the world this year, the WHO said on Friday, and Saudi authorities have deployed as many as 20,000 health workers to deal with major outbreaks during the hajj.

Marghlani said the threat of heavy rain in Mecca could raise the health risks for pilgrims, but that, "We have planned for this possibility."

Amid a war of words between Riyadh and Tehran over predominantly Sunni Muslim Saudi Arabia's treatment of Iran's mainly Shiite pilgrims, sources in the Iranian delegation said they would undertake a ritual protest in their own camp in the massive pilgrims' tent city in Mina valley.

While Saudis have banned any political acts that may disrupt the hajj, the Iranians said they will go ahead with the ritual chanting of "death to Israel, death to the United States" and call for unity among Muslims around the world.

Traffic was jammed around Mecca late Tuesday as last hundreds of thousands of latecomers arrived in the western Saudi Arabia city to launch into the rites of the hajj, required for all able Muslims at least once in their lifetime.

The rites begin with the "tawaf," the circling seven times of the cubic Kaaba building in the centre of the massive, one-million-person capacity Grand Mosque, in whose direction all Muslims around the world pray.

Pilgrims then proceed to Mina to spend the night before climbing Mount Arafat, near where Islam's prophet Mohammed gave his final sermon, to recite the Koran and pray.

Possible flooding from a rainstorm Wednesday could complicate the movement of so many people, with drainage systems limited in normally parched Saudi Arabia.

Flooding in the Mina tent city " would be a catastrophe," said Karim Saleh, 34, a pilgrim from Qatar.

Authorities were expecting about two million foreigners this year for the annual pilgrimage. But the number of pilgrims from inside the kingdom, which can top half a million, is expected to be sharply down this year due to fears about the flu.